v0.53.1: refresh recovery units on periodic cache cycle (idempotent)

CaptureRecoveryUnit now builds content in memory and skips writes when the unit
is already current (checksum + dump-set + version), so it can run from RefreshCache
(startup + every 5m) without thrashing the USB drive. Units now exist shortly after
startup and track config changes without waiting for the daily DB dump. +idempotency test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/backup/backup.go

@@ -487,6 +487,10 @@ func (m *Manager) RefreshCache(nextDBDump time.Time) {
 	// Discover app data — all deployed stacks, backup is mandatory
 	if m.stackProvider != nil {
 		status.AppDataInfo = DiscoverAppData(m.stackProvider, status.DiscoveredDBs)
+		// Phase 2: keep each app's recovery unit current with its definition. Idempotent
+		// (checksum-skip), so this periodic refresh only writes when the config actually changed,
+		// and ensures units exist shortly after startup without waiting for the daily DB dump.
+		m.captureAllRecoveryUnits()
 	}
 
 	// Fill in dynamic fields under lock.